1. Field of the Invention
The invention relates generally to methods of growing high quality gallium arsenide (GaAs) epitaxial layers by organo metallic chemical vapor deposition (OMCVD) and, in particular, to the methods which take place under less hazardous conditions than are currently employed for this technology.
2. Description of the Prior Art
The OMCVD (organometallic chemical vapor deposition) of GaAs typically involves the pyrolysis of trimethylgallium and arsine which are flowed over a GaAs substrate wafer which is/has been heated to 600.degree.-850.degree. C. in a reactor. The gases begin to decompose as they enter the heated region above the wafer, and crystalline GaAs is deposited via a combination of gas phase and surface reactions.
Due to differences in reactivity, the molar ratio of arsine to trimethylgallium (the "V/III" ratio) must be kept quite high in order to obtain n-type GaAs. Typically this "V/III" ratio is on the order of 20 or more. Also, growth temperatures must be maintained in the regime of 600.degree.-850.degree. C., with most growths carried out at 650.degree.-700.degree. C., since at temperatures below 600.degree. C., the epilayer surfaces exhibit very poor morphology. For example, at temperatures below 600.degree. C., the epilayer surfaces are coated with Ga-rich hillocks. These hillocks are undesirable because they alter the electrical properties of the surface, and the poor morphology can severely reduce device yield during processing.
Some alternative growth processes have also been reported recently. These involve the growth of GaAs by OMCVD using organoarsenic reagents in conjunction with trimethylgallium, without arsine. These organoarsenic compounds include tertiarybutylarsine (t-BuAsH.sub.2), trimethylarsenic (Me.sub.3 As), diethylarsine (Et.sub.2 AsH), and triethylarsenic (Et.sub.3 As). Nearly all of these alternative arsenic reagents were found to produce n-type GaAs films at very low V/III molar ratios (&lt;5), but these epilayers were generally found to be highly compensated, with the predominant p-type impurity being carbon (diethylarsine was found to produce some reasonable quality GaAs under certain conditions, however). The temperatures required for growth using these reagents were typically 550.degree.-800.degree. C.
There are several drawbacks to the use of arsine in the OMCVD growth of GaAs. The most significant problem is the safety risk associated with this material. Arsine gas is extremely toxic, with a lethal concentration of 3 ppm. In addition, arsine is stored in high pressure cylinders which poses a severe health threat should a leak or reactor damage occur. These combined dangers make it very difficult to get local building code approvals for the construction of OMCVD reactors in or near populated areas. Moreover, it is expensive to set up monitoring systems, scrubbers, emergency rescue gear, and related procedures for the handling of arsine.
A second important drawback is the need for high V/III ratios to produce n-type GaAs from arsine and trimethylgallium. These high ratios make this process wasteful, and also require that very large quantities of arsine be stored on site. A third major drawback is that growth temperatures are limited to &gt;600.degree. C. when using arsine, which may be unsuitable when attempting to grow GaAs on other substrate materials, such as phosphides, which may have problems with outgassing at these higher temperatures.
The main reasons for the interest in using alkylarsenic compounds are: (1) they generally eexhibit lower toxicities than arsine, and (2) less energy is required for their thermal decomposition when compared with arsine. This latter feature implies that lower growth temperatures could be used and that alkylarsines should react more efficiently with Me.sub.3 Ga such that a lower arsenic/gallium ratio would be required for GaAs growth. These alkylarsenic compounds have some significant disadvantages, however. For example, these substitutes generally do not produce high quality epilayers. The studies on GaAs growth using Et.sub.3 As/Me.sub.3 Ga have shown that the epilayers grown from these reagents alone are heavily contaminated with carbon (10.sup.17 -10.sup.21 cm.sup.-3). Carbon is also the only detactable impurity by SIMS analysis as reported in "Atmospheric Pressure OMCVD Growth of GaAs using Triethyl arsenic and Alkylgallium Precursors", Paper C4.9, Mat. Res. Soc. Fall Meeting, Boston, MA, 1987. This carbon contamination problem has also been observed in GaAs epilayers grown from several other alkylarsenic reagents. The unintentional carbon doping is apparently due to the inherent growth chemistry of the Et.sub.3 As/Me.sub.3 Ga reagent mixture, and might be attributed either to an incomplete decomposition of Et.sub.3 As such that arsenic-alkyl bonds are not entirely cleaved and a certain number of ethyl groups are incorporated into the growing GaAs film, or to the formation of large quantities of alkyl radicals in the gas phase, which can react with the substrate surface, and thus become incorporated into the epilayer.
These mechanisms would indicate that without arsine there is no method by which surface-bound alkyl groups could be removed. Most of the alternative organoarsenic compounds also produce GaAs epilayers with poor surface morphologies. Thus, there is a strong need to be able to improve the quality of GaAs epitaxial layers grown by OMCVD from these less hazardous organoarsenic materials.
It is therefore an object of the present invention to develop a safer process for growing GaAs. It is yet another object of the invention to greatly reduce the amount of arsine used in growing GaAs. It is still another object of the invention to develop a lower-temperature process for growing good quality GaAs epilayers with smooth, mirror-like surfaces. It is yet another object of the present invention to develop a process which reduces the contamination from carbon impurities which is characteristic of other contemporary processing.